Internal waves are energetic and ubiquitous in the ocean and their interactions with submarine topography could drive sediment resuspension and reshape seafloor substrate. However, the mechanisms governing these interactions are largely unknown. Here, using observational and reanalysis data at Pacific deep-sea seamounts, we show how near-inertial internal waves (NIWs) may affect substrate variability via interactions with seamounts. Taylor caps with anticyclonic vorticity generated above the seamounts can act as a "chimney" that focuses NIW energy. The NIWs can further undergo reflection at the summits and generate strong bottom currents to erode sediments. As wind energy input to NIWs increases with latitude, this process could contribute to the removal of sediments at seamount summits further north than 20°N, exposing the rock outcrops needed for biogenic structuring species and thus boosting species richness. Given that Taylor caps occur at ~93% of the global seamounts, our findings suggest a wave-topography interaction that could shape the substrate and enrich the biodiversity in the deep ocean.

Internal waves with frequency larger than twice the local minimum allowable wave frequency can be susceptible to parametric subharmonic instability (PSI). This instability draws energy from the wave and provides a mechanism for generating small-scale turbulence and mixing. In the ocean, strongly baroclinic flows at the submesoscale adjust the minimum frequency of internal waves such that it is possible for PSI to occur for locally near-inertial waves. One setting where this may occur is in baroclinic bottom boundary layers along sloping topography, where near-bottom interior flows in the sense of Kelvin-wave propagation lead to a reduction of bottom boundary layer Ertel potential vorticity, and consequently lower the minimum frequency sufficiently to allow PSI. Linear stability analysis, and nonlinear simulations, show that PSI grows at a rate determined by the vertical stratification of the bottom boundary layer, and the slope Burger number. Wave shear production is the primary energy source for the instability, with additional contributions from buoyancy production that depend on the slope parameters. A partially compensating loss of energy to geostrophic shear production becomes increasingly important as the flow approaches the marginally stable state. These results suggest PSI as a potential mechanism for generating near-bottom mixing in the ocean.

The Philippine Sea in the western Pacific is a region with high mesoscale eddy kinetic energy that is buffeted by intense typhoons. Such typhoons generate strong near-inertial waves (NIWs), making this region ideal for studying interactions between typhoon-driven NIWs and mesoscale eddies. To study such interactions, a field campaign was conducted in the Philippine Sea that targeted an anticyclonic eddy after the passage of Super Typhoon Mawar. The study was part of the US Office of Naval Research Departmental Research Initiative ARCTERX (Island Arc Turbulent Eddy Regional Exchange). During the campaign, ship and float-based velocity measurements revealed layers of intense vertical shear oscillating at slightly sub-inertial frequencies in the anticyclone. The shear layers were stronger toward the eddy center and coincided with patches of elevated turbulence. An idealized numerical simulation initialized with a symmetric eddy modeled after observations and forced by reanalysis winds was used to study the formation of NIWs by Typhoon Mawar and their interactions with the eddy. The model captured the structure and vertical propagation of the observed shear layers and demonstrated how the dynamics of the NIWs in the anticyclone are consistent with NIW trapping following the theory of zeta-refraction. The simulated shear layers were not as intense as those that were observed and could not explain the patches of enhanced turbulence. Processes not included in the model, more specifically the internal tides that are particularly strong in the Philippine Sea, likely contribute to the discrepancy. Energy exchange between the NIWs and the anticyclone diagnosed using the model output was weak, suggesting that typhoon-driven NIWs play a secondary role in the energetics of eddies in the Philippine Sea, or that the idealized nature of the model limited wave-mean flow energy exchange.

Time-varying winds blowing over an eddying ocean generate near-​inertial waves (NIWs) that tend to be trapped in anticyclones. Such anticyclones have been termed inertial chimneys in the past but have recently been renamed inertial drainpipes, given their propensity to funnel NIW energy downward to the deep ocean. Here, we present evidence of a semi-blocked inertial drainpipe where downward-​propagating NIWs trapped in an anticyclone are partially reflected off the permanent pycnocline, returned toward the surface, and dissipated at the top of the seasonal pycnocline in a submesoscale filament of anticyclonic vorticity. Observations made on the northern rim of an anticyclone in the Iceland Basin include a high-​resolution survey of velocity, hydrography, and microstructure. Upward-propagating NIWs were observed in a salty, submesoscale filament of anticyclonic vorticity near the edge of the eddy, potentially trapped there. Above the filament and at the top of the seasonal pycnocline, turbulence was enhanced over what could be attributed to local winds and surface cooling. Ray tracing suggests the filament could have channeled and focused trapped upward-propagating NIWs, acting as an inertial chimney in a truer sense of the term, possibly intensifying the wave energy sufficiently to sustain the observed turbulence. Numerical simulations of NIWs in anticyclonic vorticity and stratification representative of the observations suggest that the upward-propagating NIWs could have been generated by a wind event 12 days prior and reflected off a sharp jump in stratification at the base of the anticyclone. Here, the transition between the weakly stratified winter mixed layer and the permanent pycnocline partially reflects downward-​propagating NIWs, limiting the inertial drainpipe effect.

Over 35 years ago, the influential Ocean Storms Experiment (OSE) in the Northeast Pacific documented, for the first time, the generation of near-inertial waves (NIWs) by a storm and the subsequent radiation of the waves away from the forcing. The NIWs were observed to radiate equatorward and downward, consistent with the theory of β-refraction, which attributes such NIW propagation to the gradient in Earth’s planetary vorticity, β. Surprisingly, there was no evidence that gradients in the vorticity of mesoscale eddies in the region affected the NIWs, despite the fact that these gradients were nearly 10 times larger than β. In contrast, NIWs observed in the recent Near-Inertial Shear and Kinetic Energy in the North Atlantic Experiment (NISKINe) were strongly affected by the mesoscale eddy field in the region. In this article we explain the distinct behavior of the NIWs observed in the two experiments through a careful reanalysis of the observations, which are then interpreted using simulations and NIW-mean flow interaction theory. The observed differences can be partially attributed to how NIWs were measured in the two experiments. But more interestingly, we find that wind energy was injected primarily into low vertical modes during OSE and more broadly into higher modes during NISKINe. This, combined with the stronger stratification in the Northeast Pacific, implies that NIWs are more dispersive and hence less susceptible to being modified by vorticity there than they are in the North Atlantic.

Abstract Biological productivity in the Southern Ocean is modulated by iron availability. Every summer, a large phytoplankton bloom forms northwest of the Ross Sea, above the Antarctic Australian Ridge (AAR), due to a plume of iron‐rich waters. Here, we investigate the origin and trajectories of these iron‐rich waters by analyzing water mass observations and Lagrangian experiments. Output from the Southern Ocean State Estimate (SOSE) and in situ measurements reveal that iron‐rich AAR bloom waters share properties with Modified Circumpolar Deep Water (MCDW), which forms on the Antarctic shelf‐slope. The Lagrangian experiments are conducted using SOSE velocities. Bloom waters tracked with virtual Lagrangian particles highlight an along isopycnal pathway of MCDW from Antarctica's shelf‐slope to the AAR bloom site, illustrating advection of these waters by the Balleny Gyre. These results are supported by temperature‐salinity analyses, which show a correlation between waters advected northwards; MCDW properties; and high iron concentrations.

Along with boundary layer turbulence, downward radiation of near-inertial waves (NIWs) damps inertial oscillations (IOs) in the surface ocean; however, the latter can also energize abyssal mixing. Here we present observations made from a dipole vortex in the Iceland Basin where, after the period of direct wind forcing, IOs lost over half their kinetic energy (KE) in two inertial periods to radiation of NIWs with minimal turbulent dissipation of KE. The dipole's vorticity gradient led to a rapid reduction in the NIW's lateral wavelength via z refraction that was accompanied by isopycnal undulations below the surface mixed layer. Pressure anomalies associated with the undulations were correlated with the NIW's velocity yielding an energy flux of 310 mW m22 pointed antiparallel to the vorticity gradient and a downward flux of 1 mW m22 capable of driving the observed drop in KE. The minimal role of turbulence in the energetics after the IOs had been generated by the winds was confirmed using a large-eddy simulation driven by the observed winds.

The two-dimensional stability of vertically sheared inertial oscillations at ocean fronts is explored through a linear stability analysis and nonlinear simulations. Baroclinic effects reduce the minimum frequency of inertia-gravity waves to an extent determined by the balanced Richardson number ${{Ri}}$ of the front. Below a critical value of ${{Ri}}$ , which depends on the strength of the inertial shear, the inertial oscillations become unstable to parametric subharmonic instability (PSI) resulting in growing perturbations that oscillate at half the inertial frequency $f$ . Since the critical value is always greater than 1, PSI can occur at fronts stable to symmetric instability. Although modest in weak inertial shear, growth rates exceeding $f/2$ can be achieved for inertial shear greater than or equal to the thermal wind shear. Our formulation allows for non-hydrostatic perturbations and can be applied to initially unstratified geostrophic adjustment problems. We find that PSI will almost totally damp the transient oscillations that arise during geostrophic adjustment. The perturbations gain energy at the expense of the inertial oscillations through ageostrophic shear production. The perturbations then themselves become unstable to secondary Kelvin–Helmholtz instabilities creating a pathway by which the inertial oscillations can be dissipated rapidly. In contrast to symmetric and baroclinic instabilities that draw on a front's kinetic or potential energy, PSI acts to increase the energy stored in the balanced front as the convergence and divergence of the eddy-momentum fluxes set up a secondary circulation in the sense to stand up the front.

Recent work has shown that, when non-traditional (NT) effects associated with the horizontal component of the Coriolis parameter are taken into account, Equatorial Waves (EWs) experience critical reflection when they reflect off the seafloor at the latitude where their frequency is equal to the inertial frequency. As a result, the vertical shear associated with the wave is strongly enhanced locally and results in bottom-intensified mixing. Using an off-the-shelf parameterization for mixing, these studies have shown that this process could play an important role in driving diapycnal upwelling in the abyssal ocean, but the specific mechanisms generating the mixing have not been studied yet. In this work, we address this limitation by running two-dimensional, high-resolution, non-hydrostatic simulations of the critical reflection of internal waves modified by NT effects. These simulations can resolve the instabilities triggered when the wave reflects off the bottom, allowing us to characterize the energy cascade to smaller scales and to estimate the mixing it generates. We find that shear instabilities drive elevated turbulent diffusivities between 10−1–10−3 m2/s over a critical layer of 100-300 m thick. The shear instabilities result directly from the enhancement of kinetic energy in the reflected wave that is confined against the seafloor during the critical reflection process. Simultaneously, higher harmonics are generated and flux energy upwards in the water column. These higher harmonics are unstable to parametric subharmonic instability, which absorbs their energy and drive enhanced dissipation above the critical layer, to a height of O(1000 m) off the bottom. We show how these results depend on key elements of the EWs and of the medium, and discuss the implementation of a parameterization of these effects in global ocean models.

Studies of internal wave-driven mixing in the coastal ocean have been mainly focused on internal tides, while wind-driven near-inertial waves (NIWs) have received less attention in this regard. This study demonstrates a scenario of NIW-driven mixing over the Texas-Louisiana shelf. Supported by a high-resolution simulation over the shelf, the NIWs driven by land-sea breeze radiate downward at a sharp front and enhance the mixing in the bottom boundary layer where the NIWs are focused due to slantwise critical reflection. The criterion for slantwise critical reflection of NIWs is (where ω is the wave frequency, S bot is the bottom slope, and S p is the isopycnal slope) under the assumption that the mean flow is in a thermal wind balance and only varies in the slope-normal direction. The mechanism driving the enhanced mixing is explored in an idealized simulation. During slantwise critical reflection, NIWs are amplified with enhanced shear and periodically destratify a bottom boundary layer via differential buoyancy advection, leading to periodically enhanced mixing. Turbulent transport of tracers is also enhanced during slantwise critical reflection of NIWs, which has implications for bottom hypoxia over the Texas-Louisiana shelf.

Our current knowledge of the lifecycle of the most energetic surface-generated internal waves in the ocean, wind-driven near-inertial waves (NIWs), is reviewed. The review covers the three stages in a NIW's lifecycle: formation as near-inertial motions in the mixed layer by the winds, propagation into the ocean interior as a NIW, and demise through turbulent dissipation and mixing, wave-wave interactions, and/or absorption into mean flows. A main goal of the review is to provide an update of our understanding of the processes that influence the wind-work on near-inertial motions, and those that govern the interactions of NIWs with mesoscale eddies and fronts. The impacts of NIWs on ocean mixing, both in the vertical and horizontal, are discussed.

Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region's dead zone.

Submesoscale fronts with large horizontal buoyancy gradients and $O(1)$ Rossby numbers are common in the upper ocean. These fronts are associated with large vertical transport and are hotspots for biological activity. Submesoscale fronts are susceptible to symmetric instability (SI) – a form of stratified inertial instability which can occur when the potential vorticity is of the opposite sign to the Coriolis parameter. Here, we use a weakly nonlinear stability analysis to study SI in an idealised frontal zone with a uniform horizontal buoyancy gradient in thermal wind balance. We find that the structure and energetics of SI strongly depend on the front strength, defined as the ratio of the horizontal buoyancy gradient to the square of the Coriolis frequency. Vertically bounded non-hydrostatic SI modes can grow by extracting potential or kinetic energy from the balanced front and the relative importance of these energy reservoirs depends on the front strength and vertical stratification. We describe two limiting behaviours as ‘slantwise convection’ and ‘slantwise inertial instability’ where the largest energy source is the buoyancy flux and geostrophic shear production, respectively. The growing linear SI modes eventually break down through a secondary shear instability, and in the process transport considerable geostrophic momentum. The resulting breakdown of thermal wind balance generates vertically sheared inertial oscillations and we estimate the amplitude of these oscillations from the stability analysis. We finally discuss broader implications of these results in the context of current parameterisations of SI.

In Part 1 (Wienkers, Thomas & Taylor, J. Fluid Mech. , vol. 926, 2021, A6), we described the theory for linear growth and weakly nonlinear saturation of symmetric instability (SI) in the Eady model representing a broad frontal zone. There, we found that both the fraction of the balanced thermal wind mixed down by SI and the primary source of energy are strongly dependent on the front strength, defined as the ratio of the horizontal buoyancy gradient to the square of the Coriolis frequency. Strong fronts with steep isopycnals develop a flavour of SI we call ‘slantwise inertial instability’ by extracting kinetic energy from the background flow and rapidly mixing down the thermal wind profile. In contrast, weak fronts extract more potential energy from the background density profile, which results in ‘slantwise convection.’ Here, we extend the theory from Part 1 using nonlinear numerical simulations to focus on the adjustment of the front following saturation of SI. We find that the details of adjustment and amplitude of the induced inertial oscillations depend on the front strength. While weak fronts develop narrow frontlets and excite small-amplitude vertically sheared inertial oscillations, stronger fronts generate large inertial oscillations and produce bore-like gravity currents that propagate along the top and bottom boundaries. The turbulent dissipation rate in these strong fronts is large, highly intermittent and intensifies during periods of weak stratification. We describe each of these mechanisms and energy pathways as the front evolves towards the final adjusted state, and in particular focus on the effect of varying the dimensionless front strength.

Tropical storms and hurricanes frequently pass over the Straits of Florida, energizing the near‐inertial wave (NIW) field in the strait. Two ship‐based surveys, which were launched shortly after storms, observed velocity shear bands over the western slope of the strait—clear signatures of NIWs. Also, the hydrographic measurements demonstrate the formation of a slantwise critical layer (where isopycnals are parallel with bathymetry) over the western slope, which is known to be a trapping zone for NIWs. A realistic simulation of the Straits confirms the emergence of NIWs under a tropical storm, which is accompanied by inertially modulated bottom‐enhanced mixing over the western slope. The mechanism driving the mixing is that the storm‐generated NIWs radiate downward from the core of the Florida Current, reflect off the eastern slope, and enter the slantwise critical layer over the western slope; upon entering, wave trapping, and amplification lead to the enhanced mixing.

Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre.

At low latitudes in the ocean, the deep currents are shaped into narrow jets flowing eastward and westward, reversing periodically with latitude between 15°S and 15°N. These jets are present from the thermocline to the bottom. The energy sources and the physical mechanisms responsible for their formation are still debated and poorly understood. This study explores the role of the destabilization of intra-annual equatorial waves in the jets’ formation process, as these waves are known to be an important energy source at low latitudes. The study focuses particularly on the role of barotropic Rossby waves as a first step toward understanding the relevant physical mechanisms. It is shown from a set of idealized numerical simulations and analytical solutions that nonlinear triad interactions (NLTIs) play a crucial role in the transfer of energy toward jet-like structures (long waves with short meridional wavelengths) that induce a zonal residual mean circulation. The sensitivity of the instability emergence and the scale selection of the jet-like secondary wave to the forced primary wave are analyzed. For realistic amplitudes around 5–20 cm s −1 , the primary waves that produce the most realistic jet-like structures are zonally propagating intra-annual waves with periods between 60 and 130 days and wavelengths between 200 and 300 km. The NLTI mechanism is a first step toward the generation of a permanent jet-structured circulation and is discussed in the context of turbulent cascade theories.

Recent theoretical work has shown that, when the so-called non-traditional effects are taken into account, the reflection of Equatorially Trapped Waves (ETWs) off the seafloor generates strong vertical shear that results in bottom-intensified mixing at the inertial latitude of the ETW via a mechanism of critical reflection. It has been estimated that this process could play an important role in driving diapycnal upwelling in the Abyssal Meridional Overturning Circulation (AMOC). However, these results were derived under an idealized configuration with a monochromatic ETW propagating through a flat ocean at rest. To test the theory in a flow that is more representative of the ocean, we contrast a set of realistic numerical simulations of the Eastern Equatorial Pacific run using either the hydrostatic or quasi-hydrostatic approximation, the latter of which accounts for non-traditional effects. The simulations are nested into a Pacific-wide hydrostatic parent solution forced with climatological data and realistic bathymetry, resulting in an ETW field and a deep circulation consistent with observations. Using these simulations, we observe enhanced abyssal mixing in the quasi-hydrostatic run, even over smooth topography, that is absent in the hydrostatic run. The mixing is associated with inertial shear that has spatio-temporal properties consistent with the critical reflection mechanism. The enhanced mixing results in a weakening of the abyssal stratification and drives diapycnal upwelling in our simulation, in agreement with the predictions from the idealized simulations. The diapycnal upwelling is on the order of O(10) Sv and thus could play an important role in closing the AMOC.

This study describes a specific type of critical layer for near-inertial waves (NIWs) that forms when isopycnals run parallel to sloping bathymetry. Upon entering this slantwise critical layer, the group velocity of the waves decreases to zero and the NIWs become trapped and amplified, which can enhance mixing. A realistic simulation of anticyclonic eddies on the Texas-Louisiana shelf reveals that such critical layers can form where the eddies impinge onto the sloping bottom. Velocity shear bands in the simulation indicate that wind-forced NIWs are radiated downward from the surface in the eddies, bend upward near the bottom, and enter critical layers over the continental shelf, resulting in inertially-modulated enhanced mixing. Idealized simulations designed to capture this flow reproduce the wave propagation and enhanced mixing. The link between the enhanced mixing and wave trapping in the slantwise critical layer is made using ray-tracing and an analysis of the waves' energetics in the idealized simulations. An ensemble of simulations is performed spanning the relevant parameter space that demonstrates that the strength of the mixing is correlated with the degree to which NIWs are trapped in the critical layers. While the application here is for a shallow coastal setting, the mechanisms could be active in the open ocean as well where isopycnals align with bathymetry.